Anti-backlash circuit for hydraulic drive system



March 8, 1966 JAMES E. WEBB 3,238,730

ADMINISTRATOR OF THE NATIONAL AERoNAuTIcs AND SPACE ADMINISTRATIONANTI-BACKLASH CIRCUIT FOR HYDRAULIC DRIVE SYSTEM Filed Feb. 5, 1965 4Sheets-Sheet 1 dame: fl. Accra INVENTOR BY q 1% g z vzzzwp ATTORNEYSMarch 8, 1966 JAMES E. WEBB 3,238,730

ADMINIsTRAToR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONANTI-BACKLASH CIRCUIT FOR HYDRAULIC DRIVE SYSTEM Filed Feb. 5, 1965 4Sheets-Sheet 2 10/417 AXLE ATTORNEYS March 8, 1966 JAMES E B 3,238,730

ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONANTI-BACKLASH CIRCUIT FOR HYDRAULIC DRIVE SYSTEM Filed Feb. 5, 1965 4Sheets-Sheet 5 dame: Z7. Ace/a INVENTOR.

ATTOR/VE YJ March 8, 1966 JAMES E. WEBB 3,238,730

AUTICS CIRCUIT FOR HYDRAULIC DRIVE SYSTEM ADMINISTRATOR OF THE NATIONALAERON AND SPACE ADMINISTRATION ANTI-BACKLASH Filed Feb. 5, 1965 4Sheets-Sheet 4 10/15 AXLE dame: D. Ace/c2 I NVENTOR.

BY SQ ATTORNEYS United States Patent 3,238,730 ANTI-BACKLASH CIRCUIT FORHYDRAULIC DRIVE SYSTEM James E. Webb, Administrator of the NationalAeronautics and Space Administration, with respect to an invention ofJames D. Acord Filed Feb. 5, 1965, Ser. No. 430,780 Claims. (Cl. 60-97)This invention relates to hydraulic power means for accurately andresponsively controlling the movement of large loads such as a deepspace tracking antenna. More specifically, it provides for theelimination of backlash in the gear trains of hydraulic motors .by theuse of an improved hydraulic circuit and alternating valve meanstherein.

In order to move a body having large weight or mass, it is a commonexpedient to use a gear train or other similar system which necessarilyemploys the principle of mechanical advantage. In so doing there isintroduced an intermediate power transmitting set of machine elementsbetween the driving body or prime mover and the load being driven. Dueto imperfect inter-engagement between these power transmitting elements,the movement of the load is not immediately responsive to the startingof the prime mover. Thus there results a debilitating effect upon loadcontrol. This adverse effect is compounded .by the well-known problem ofgear train backlash which results from the spaced engagement or playbetween inter-engaging teeth of adjacent gears or other drive elements.This may occur when the driven load moves ahead of the drive means, asmay occur in a strong wind, when the load is initially moved from astatic condition, or when there is a change in the applied power ordirection. The spaced engagement of gear teeth may result from severalcauses such as gear production techniques or normal operational wear.Since the latter is the more predominant cause of gear backlash andnon-responsive load movement, certain prior art approaches to thesecommon control problems have embodied means to create a drag on thedrive gears and thereby constantly keep them in positive engagement,regardless of their degree of wear. An example of the latter, more fullydescribed herein, is the dual drive, series opposing system in which onemotor is back loaded against the other.

In relatively sophisticated machinery, such as large space antennaswherein it is necessary to track the path of probes millions of miles inspace, extreme accuracy and control of the antenna is imperative, andthus the problem is even more acute. However, due to the oscillatorytype movements of such antennas, it is apparent that the gear and drivesystems are subject to severe wear which in turn causes the imperfectfit primarily responsible for the aforementioned backlash. As a resultof the backlash, the accuracy of the antenna and the control over it areimpaired.

Briefly described, this invention provides for a pair of hydraulic drivemotors connected in parallel relation with respect to a common fluidfeed line. The two motors are cooperatively connected to a bull or loadgear by means of separate power transmitting gear trains. There isadditionally provided a hydraulic circuit having valve control and pumpmeans therein so as to selectively vary the pressure and change thedirection of the hydraulic fluid flowing therethrough. A pair ofadjustable pressure reducing valves are also disposed in the circuit insuch a manner as to produce a pressure drop to one motor at a time,thereby causing that motor to have a lower torque output than theprimary drive motor. In this manner backlash is not only eliminated fromthe gear system because of the constant drag applied by the lowertorqued motor but, in addition, the magnitude of the drag forces3,238,730 Patented Mar. 8, 196.6

ice

may be increased or decreased as desired. As a result of the reduceddrag forces, there is less stress on machine parts, therefore resultingin longer life and less maintenance. Also, due to the reduced hydraulicpressures in the system, there is more responsive control over the load.For example, in tests of the system disclosed herein, a deep spacetracking antenna was moved repeatedly in increments as small as .005degree, or about 15 seconds of arc in either direction, whereas in priorsystems no movement less than .02 to .03 degree could be accomplished.

The foregoing and still other advantages and features of this improvedsystem will become more fully apparent from the following detaileddisclosure and drawings wherein like numerals denote like elements inthe various views and wherein:

FIG. 1 is a schematic representation in partial crosssection of aconventional prior art hydraulic rotary type drive system having noanti-backlash provision.

FIG. 2 is an enlarged view of the motor and valve portion shown in thehydraulic circuit of FIG. 1.

FIG. 3 shows a schematic representation in partial crosssection of aprior art anti-backlash circuit which incorporates the commonly usedseries-opposing motor arrangement. Such a system embodies the samegeneral type of servo valve shown in FIG. 2.

FIG. 4 is a schematic representation in partial crosssection of thehydraulic drive circuit embodying the improved anti-backlash provisionwhich is the subject of this invention.

FIG. 5 shows an anlarged view of the simplified counterbalance typevalve used in the circuit of FIG. 4.

FIG. 6 shows a schematic representation in partial crosssection of thedrive circuit of the invention when under operating conditions.

Reference is made to the conventional hydraulic drive system of FIG. 1.The circuit comprises a fluid reservoir 1 wherein there is stored thehydraulic fluid at atmospheric pressure for operating the system. Sincethe circuit is a closed loop, the reservoir contains a return line 2 andan out line 3. The out line is connected to pump 4 which may be ofconventional variety. Supply line 5 feeds hydraulic fluid from the pumpinto a four-way servo valve 6 which serves to directionally operate thedrive motion of the system. There is provided a pair of fluid lines 7, 8connected to opposing sides of the servo valve and to a hydraulicallydriven motor 9.

Mechanically connected and engaged with the hydraulic motor in awell-known manner is the gear train 10 and load carrying bull gear 12.schematically indicated at the bottom of the motor is a case drain line13 which communicates with the return line 2 from the servo valve. Casedrain 13 serves to prevent internal motor leakage from building upexcessive pressure within the case. Lacking such provision, a buildup ofpressure could cause external leakage or rupture of the motor casing.There is usually provided a valve of conventional design (not shown) forregulating the leakage flow through this line.

By use of conventional external control, pump 4 creates a systempressure P, in supply line 5. Likewise, through external controls, servovalve 6 is caused to rotate in the desired direction and thus direct thefluid into the motor. The degree of fluid pressure into motor 9 isdependent on and proportional to the opening resulting from rotation ofthe valve. If the valve 6 were rotated from its closed position to 22degrees beyond the horizontal, as shown in FIG. 2, it is apparent thatthe vanes 18 of the motor would rotate slower and hence result in lesstorque on the gear train than if the valve were thrown full openthat is,if the valve were rotated sufliciently to present the fullcross-sectional area of fluid line 7 to the valve passageway 24. Thuscontrol over the load is affected by appropriate movement of the servovalve. The quality of the control, however, is adversely affected by thepresence of backlash in the gear train. As may be visualized byexamination of FIG. 1, a backlash situation in gear train 10 may becreated upon initial movement of servo valve 6 from the horizontalposition because the gears may be in disengagement such as when startingfrom static condition, or when changing direction. One prior artsolution to this problem in relatively small gear trains resulted insplitting the gears at each mesh, and spring loading each split gearagainst the other. In larger gear trains and heavier loads, such anarrangement is not feasible due to the magnitude of forces involved.

A prior art arrangement designed for the elimination of backlash in thedrive system for large loads is referred to as the dual motor seriesopposing system. Although the dual motor series opposing drive systempresents a limited remedy to the problem of backlash, it has been foundto introduce factors which not only detract from optimal control of theload but which will, in some instances to be explained hereinafter,preclude effective control over it. In the series system shown in FIG.3, servo valve 6, hydraulic motors M M and gear trains 10 and 26 operatein the same manner as the valve, motor, and train described previously.

The dual drive-series opposing system is arranged so that one gear trainacts as a drag on the bull gear, while the other gear train is drivingthe bull gear. When the load direction is changed, these two gear trainsexchange functions, that is, the prior drag train now drives, and theprior drive train becomes a drag. In this manner the gear teeth in thedriving gear train are always kept in positive engagement and backlashis substantially reduced. An examination of this circuit arrangementshows that with the servo valve 6 in a closed position (as in FIG. 1),the system is in a static condition and a pressure I exists in thesupply line from the pump. Clockwise movement of the servo valve to aposition shown by the dotted lines allows fluid to flow, thus producingan initial pressure P, in the fluid line 7. Neglecting frictional lossesoccurring from fluid flow, the same pressure P is seen by port A goinginto motor M The pressure P, is exerted on the vanes (not shown) ofmotor M causing it to rotate clockwise and thus causing it to have aclockwise torque TM,. The torque TM, is transmitted to and through geartrain 10 to bull gear 12 (or load) which then moves in a clockwisedirection. As the bull gear rotates, a torque is applied to and throughthe train 26 which in turn is applied to .motor M causing it to rotate.This causes M to act as a pump with respect to the fluid flowingtherethrough since no effective pressure exists at its port A Thus, asthe bull gear rotates, driving motor M the fluid which exits port B ispumped into conduit 15 and through the servo valve into conduit 46 andthen into the return line 2 which leads back to the reservoir. It isseen that if the servo valve is turned to supply fluid into line 15instead of line 7, that the load will reverse its direction. Otherwisethe system will operate as described above.

As with the hydraulic motor described in FIG. 1, there are provided casedrain conduits 11, 25 for each of the motors M M These conduits, andconnecting conduit 27, provide a path for leakage flow from either motorto reduce internal motor pressure. The connecting conduit transfers thefluid back to the reservoir via return line 2.

With the above circuit arrangement it is seen that the teeth ofinter-engaging gears in gear train 10 are kept in positive contactbecause the bull gear cannot coast ahead of the gear train 10 due to thedrag exerted on it by the train 26. It is plausible, of course, thatsome external force acting on the load, such as wind, etc., could exceedthe drag forces created by gear train 26 and thus produce a backlash inthe drive system of motor M Although the constant drag effect in thissystem builds an antibacklash provision into its operation, there iscreated, as a result, a series of operational problems and deficiencies.

One of these problems is the constant high magnitude of stress presentat contact points of interengaging gear teeth, the source of which isthe added load (or drag forces) which the drive train must move. Anotherproblem is the absence of means for effectively reducing the resistanceor drag forces when they are not needed. It is found that the magnitudeof these forces causes gear teeth to wear quickly and sometimes even tofracture the gear boxes. Still another problem caused by thisantibacklash arrangement is reduction in load control caused by the highsystem pressures necessary for movement.

The subject invention discloses an improved hydraulic circuit which notonly reduces the drive forces required, but which also provides a meansfor creating an effective adjustable drag-producing antibacklashprovision during operation. As a result of this improved antibacklashprovision, control over the load is significantly improved and wear onthe drive machinery greatly reduced. In this regard reference is made tothe improved system of the invention, as shown in FIG. 4, wherein a pairof fluid motors M and M are placed in parallel feed relation withrespect to the source of fluid flow of the system by providing a fork inline 7 having two conduits 30, 31 leading to the motors M and M,respectively. The pressure in these conduits would thus be the same. Inorder to create a pressure drop, however, into one of the motors andthus create less torque by that motor, counterbalance valves 33, 45(i.e. pressure reducing valves) are placed in the lines in such a mannerthat the lower pressure would be impressed across only one motor at atime, this depending upon which way the valve 6 is turned. Thus if thefluid is moving clockwise in the circuit, a pressure drop would becreated in the direction of motor M i.e., across motor M while if itwere moving counterclockwise, the pressure drop would exist across motorM Additionally, the pressure reducing valves 33, 45 may be adjusted tovary the extent of the pressure drop, the purpose being to vary thetorque on one of the motors. In such an arrangement the drag of thelower torqued motor and its associated gear train may be adjusted toaccommodate the conditions under which the machine is operating. This isaccomplished by the use of an adjustable setting 35, 36 on each of thevalves. Also, the valves are so constructed that fluid may flowtherethrough in either direction. Examination of FIG. 5 shows that fluidentering through port 56 may flow freely through the valves with nosignificant drop in pressure. Should the fluid flow through the valve inthe opposite direction however, there is a pressure drop. A detailedexplanation of the valve mechanism is set forth hereinafter.

The conduit 31 of FIG. 4 is connected to the port A of motor M so as toconstitute either a feed or return line for the motor, depending uponwhich direction the fluid is moving. To the port B is connected conduit34 which leads to a counter-balance valve 45. The counterbalance valvein turn is connected by conduits 38, 39 to the servo valve mechanism.The conduits for the motor M is thus of closed loop design with fluidsupplied from and returned to the servo valve.

A second closed loop conduit is provided for the motor M and consists ofline 39 having a fork therein forming the conduit 15 and theaforementioned conduit 38. Conduit 15 is connected to the port B ofmotor M and acts as a supply or return line for such motor, dependingupon which direction the system is moving. Connected to the port A ofthe motor is conduit 43 which leads in to the counterbalance valve 33.The valve 33 may work as a pressure reducing valve or as a mere fluidconduit in the same manner as its cooperating valve 45. As previouslyexplained, only one valve acts as a pressure re ducer at a given timeand this depends upon the direction of system movement. The closed loopconduits for both motors M and M may hereinafter be referred to as afirst circuit.

In addition to the ports of the servo valve which receive the conduits 7and 39 of the first circuit, there is provided a port each for thesupply line 5 and return line 2, both of which constitute a portion of asecond circuit. These lines conduct fluid to the first circuit andremove it therefrom, respectively, by operation of the conventionalservo valve 6. The return line 2 carries hydraulic fluid from the firstcircuit by means of its connection to an exit conduit 46 leading fromthe servo valve. The fluid reservoir 1, to which the other end of returnline 2 is connected, is at atmospheric pressure by reason of a breathertube 75 therein. Also connected to reservoir 1 is the out line 3 whichsupplies fluid to the pump 4. This fluid is then delivered from the pumpunder pressure P to the servo valve where it enters the first circuiteither through the conduits 7 or 39, depending upon which way the servovalve is turned. It is apparent, of course, that fluid flow isunidirectional in the second circuit,'while in the first circuit flowmay be either clockwise or counterclockwise. i

As noted above, a counterbalance valve is disposed in each loop of thefirst circuit. These valves 33, 45 are, as shown in FIG. 5, essentiallya check valve in parallel relation with a pressure reducing valve, thepurpose of the arrangement being to create a pressure drop in onedirection through the counterbalance device while effecting flowstoppage in the opposite direction. More specifically, it is seen thatthe valve consists of a ballcheck type arrangement with one ball 66spr'mg loaded to a closed (dotted) position. The spring constant in thespring 60 may be varied by adjusting the set screw 35 so as toaccomplish the desired drop in pressure.

The free flow side 52 comprises a similar ball-check type arrangement inwhich the ball 64 rests against a shoulder pin 63 when in the free flowposition (dotted lines). Thus, when the valve is causing a pressure dropacross its ports 58, 56 the flow of fluid into port 58 causes port 62 tobe blocked by the ball 64 while the cooperating ball 66 above port 58 isurged upward when fluid pressure exceeds the opposing compressive forceof spring 60. The movement of ball 66 is, of course, dependent on theposition of the adjustable setting 35 which thus causes the desiredpressure drop AP.

When fluid is flowing in the opposite direction, that is in through port56, the ball 66 on the pressure reducing side is forced downward by thespring 60, thus closing off fluid flow to port 58. Ball 64, however, ismoved by fluid pressure on to shoulder pin 63, thereby presenting anunobstructed or free fluid flow path at the side 52 from port 56 to port58.

For the purpose 'of describing the operation of the system shown in FIG.4 it will be assumed that a system pressure of 4000 p.s.i., exists inthe supply line 5 from the pump. Also, it is assumed that the setting Pon the counterbalance valves is 500 psi. and that load 12 is a largespace probe tracking antenna which is to move initially in a clockwisedirection. If the system is to start from a static condition, the servovalve is opened clockwise to a full open position since maximum torquewill be required to accelerate the load. Disregarding frictional losses,the pressure a port A and in line 30 will be 4000 psi. The pressure inline 43 and hence at port A of motor M is 3500 p.s.i. In the prior artdevices, motor M was required to move the load by itself since motor Mwas contributing substantially no effective torque, but was insteadacting as a drag to prevent backlash. In the subject device, valve 33 isadjusted so that M is acting to lessen or increase the drag of the geartrain 26, depending on the needs at the time. If a small drag isrequired, the valve 33 may be opened wide at setting 35, thereby causinghigh fluid pressure and applied torque by motor M Oppositely, M couldhave more drag if the valve opening were reduced by reducing its openingat 35. The adjustable torque feature makes possible the use of smallermotors both at M and M since less torque is required by each motor.Since there is less drag on the main drive motor M the wear andmaintenance is substantially reduced. In following the fiuid flowthrough the above system it is seen that fluid leaving port B flowsunder substantially reduced pressure through the free flow side 52 (FIG.5) of counterbalance valve 45 and then into line 38. The fluid leavingport B is likewise under substantially reduced pressure as it flowsthrough line 15 where it meets fluid from line 38 and returns to theservo valve via line 39. The fluid then traverses the servo valve andreturns to the reservoir 1 through return line 2 of the second circuit.From the reservoir the fluid is recycled into the system by a hydraulicpump 4.

It is clear that should the servo valve be rotated in a counterclockwisedirection there will be a pressure drop across valve 45 which causesmotor M to take up the antibacklash function, thus making motor M theprimary driver. The flow through the valves, motors, and line circuitmay be traced back to the reservoir in a counterclockwise manner whichis functionally identical to the clock: wise fluid path described above.

When the system is at rest, there wil normally be sufli: cient leakageflow through motor ports B and A and back through case drain conduit 50to maintain the pressure differentials across counterbalance valves 33and 45 and thus maintain opposing torques from motors M and M If thisleakage flow is insufficient, small adjustable orifice valves 51 and 53may be installed as shown to provide the required mount.

Also, 'as noted above, movement of the servo valve (clockwise) to aposition less than full open will cause the pressure in line 7 toincrease and the pressure in line 39 to decrease. By controlled movementof the servo valve and controlled counterbalance valve settings itbecomes apparent that relatively fine control may be exercised overmotor port pressures which in turn provide a more precise control overthe system. Once the system is operating under normal loads, that is, noacceleration and no external forces such as winds acting on the mainload, the operational torques are significantly reduced below thoserequired for acceleration. These conditions may be exemplified by thepressures shown in FIG. 6, wherein, it is seen'that due to the positionof the servo valve a pressure in the line and therefore at port A ofonly 2620 psi. exists. In that the servo valve is not fully open to thereturn line, port B is at a pressure substantially greater thanatmospheric, namely at 1380 p .s.i. The 500 p.s.i. setting on thecounterbalance valve 33 causes a pressure at port A of 2120 p.s.i. whilethe exit pressure (port B is 1380 p.s.i., the same as in the otherreturn lines of the system. The counterbalance valve 45 is operating infree flow position and the main return line 22 is at atmosphericpressure due to breather opening in the reservoir. It is found that thebull gear teeth 81 in a system under the above conditions bear atangential load of about 800 lbs. due to the low system loading. Thiscompares with loads of between 10 and 15 times as great in prior artsystems when under the same operating conditions. It should be notedthat although fluid motor M has a net positive differential pressureacross it (clockwise drive), the resulting torque is insuflicient toovercome both its internal friction and that of gear train 26, and itstill places a small net drag on the drive motor M and train 10, thusmaintaining a desirable backlash free condition. More importantly,however, is the ability to regulate by means of the counterbalancevalves the magnitude of drag which this motor exerts. Thus if the motorM is the prime driver, the load is moving clockwise and M is exerting ananti-backlash effect. If a prevailing wind exerts a counterclockwiseforce against the load movement, there could result a backlash in thesystem since the wind force in combination with drag force of M mayovercome the effective torque of the prime driver M However, with theability to vary the drag torque (by use of the settings AP), thereresults a change in the effective drive torque. By increasing thepressure drop across the counterbalance valve associated with the dragtrain the probability of setting up a backlash condition issubstantially reduced. In the opposite sense it is seen that if aprevailing wind exists in the same direction as the load is moving, thenit may be necessary to decrease the pressure drop into the drag motorsince less drag will be required to maintain a backlash free condition.This may, of course, require a simultaneous increase in pressure tomotor M It will be recognized by those skilled in the art that thesecounterbalance valves may be interconnected with each other and withexternal sensing means which control them, in accordance with theconditions present.

It is thus seen that the subject invention provides improved operatingcharacteristics regardless of whether it is under maximum torque ornormal torque requirements. During conditions of low torque operation,such as that which is encountered during the tracking of deep spaceprobes, the gear teeth and structure loads are subjected to less than1000 lbs., or about of the maximum which the drive system can repeatedlyendure. In the prior art devices the gear teeth and support structurewere subjected to loads approaching 75% of maximum. As a result of thelower forces it is apparent, as initially pointed out, that wear onmoving parts is greatly reduced. Likewise, it is apparent that due tolower friction and loading forces the accuracy and sensitivity ofcontrol of the antenna is improved and it can be moved with much lesssignal to the servo valve than was possible before.

Having thus described the nature and operation of this invention, itwill be understood that said invention permits of various modifications,alterations, and substitutions, particularly with respect to individualcomponents and their design, and also it is to be recognized that thedrive system disclosed herein is applicable to and may be embodied inapparatus other than the exemplary antenna, all without departing fromthe essence of the invention and within the scope of the claimsfollowing hereafter.

Therefore what is claimed is:

1. A reversible drive system substantially free of backlash foraccurately controlling movement of a load driven thereby comprising:

dual hydraulically operated drive motors operatively engaged with theload;

means for selectively varying the torque of one of said motors withrespect to the other so that the lower torqued motor acts as a variabledrag on the driven load, said means including a hydraulic circuit havinga primary and secondary control means therein, said primary controlmeans operatively disposed in the circuit for selectively controllingthe direction and quantity of fluid flow, and said secondary controlmeans includes an adjustable fluid pressure reduction means disposed inthe supply line of the lower torqued motor.

2. The reversible drive system of claim 1 wherein the supply line ofeither of said motors serves as a return line for its respective motorwhen the direction of fluid flow is reversed by selective control ofsaid primary control means.

3. .The drive system as recited in claim 2 wherein the supply line ofthe lower torqued motor further includes means for allowing the freeflow of hydraulic fluid in a direction opposite to the direction of thesupply fluid when said line is performing the function of a return line.

4. The reversible drive system of claim 3 wherein the means providingfor free flow of hydraulic fluid in a direction opposite to said supplyfluid direction also precludes fluid flow in the latter direction.

5. The reversible drive system of claim 3 wherein said means providingfor free flow of hydraulic fluid and said fluid pressure adjustmentmeans comprises an integral counterbalance valve.

6. The reversible drive system of claim 5 wherein said counterbalancevalve has dual passages with a check valve in one passage and a variablespring-loaded valve in the other.

7. A pair of reversible, hydraulic drive motors operatively engaged witha common load and having a fluid transmission and control systemconnected thereto for effecting improved control over movement of saidload, said fluid transmission and control system comprising:

a first closed loop conduit having one of said motors hydraulicallyconnected therein;

a second closed loop conduit having the other of said motorshydraulically connected therein;

control means common to both said conduits for selectively directingfluid through each in either of two directions;

said control means operatively connected to said conduits to drive saidmotors in the same relative direction;

an adjustable pressure reducing means disposed in each closed loopcircuit;

one of said pressure reducing means disposed to create a pressure dropinto one of said motors when fluid flows in a first direction, and theother of said pressure reducing means disposed to create a pressure dropinto the other of said motors when fluid flows in the other direction;and

means for supplying fluid from a reservoir to said control means and forreturning fluid from said motors to the reservoir.

8. The control system of claim 7 including means connecting said motorsfor selectively adjusting the pressure differential between them whensaid motors are in a static condition.

9. A reversible drive system substantially free of backlash foraccurately controlling movement of a load driven thereby comprising:

dual hydraulically operated drive motors operatively engaged with theload;

means for selectively varying the torque of one of said motors withrespect to the other so that the lower torqued motor acts as a variabledrag on the driven load, said means including a hydraulic circuit havinga primary control means operatively disposed in the circuit forcontrolling the direction and quantity of fluid flow, said hydrauliccircuit including a fluid reservoir at a pressure substantially belowthat in the supply lines to said motors;

supply and return lines, each connected to said primary control meansfor conveying hydraulic fluid to and from said hydraulic circuit;

pump means in said supply line for providing pressurized fluid to saidprimary control means;

a common case drain line connecting each of said motors to the returnline of said hydraulic circuit; and

regulating means in said case drain line of each motor for independentlyadjusting the leakage of pressure from said motors.

10. A pair of reversible, hydraulic drive motors operatively engagedwith a common load and having a fluid transmission and control systemconnected thereto for effecting improved control over movement of said-load, said fluid transmission and control system comprising:

a first closed loop conduit having one of said motors hydraulicallyconnected therein;

a second closed loop conduit having the other of said motorshydraulically connected therein;

primary control means having a pair of fluid ports one of said portscommon to each of said closed loop conduits and having a pair of otherports connected to supply and return conduits respectively;

said pair of fluid ports operatively connected to said closed loopconduits in a manner enabling said m0- tors to be driven in the samerelative direction; an adjustable pressure reducing means disposed ineach said closed loop conduits;

one of said pressure reducing means disposed to create a pressure dropinto one of said motors when fluid flows in a first direction, and theother of said pressure reducing means disposed to create a pressure dropinto the other of said motors when fluid flows in the other direction; afluid reservoir having said return conduit and said supply conduitconnected thereto; and a hydraulic pump disposed in said supply conduitfor providing fluid to said primary control means under pressure tothereby drive said motors.

References Cited by the Examiner UNITED STATES PATENTS References Citedby the Applicant UNITED STATES PATENTS 1,918,180 7/1933 Carter.1,956,369 4/1934 Woods. 2,461,877 2/ 1949 Brereton. 2,931,035 3/ 1960Reinhard et al.

EDGAR W. GEOGHEGAN, Primary Examiner.

1. A REVERSIBLE DRIVE SYSTEM SUBSTANTIALLY FREE OF BACKLASH FORACCURATELY CONTROLLING MOVEMENT OF A LOAD DRIVEN THEREBY COMPRISING:DUAL HYDRAULICALLY OPERATED DRIVE MOTORS OPERATIVELY ENGAGED WITH THELOAD; MEANS FOR SELECTIVELY VARYING THE TORQUE OF ONE OF SAID MOTORSWITH RESPECT TO THE OTHER SO THAT THE LOWER TORQUED MOTOR ACTS AS AVARIABLE DRAG ON THE DRIVEN LOAD, SAID MEANS INCLUDING A HYDRAULICCIRCUIT HAVING A PRIMARY AND SECONDARY CONTROL MEANS THEREIN, SAIDPRIMARY CONTROL MEANS OPERATIVELY DISPOSED IN THE CIRCUIT FORSELECTIVELY CONTROLLING THE DIRECTION AND QUANTITY OF FLUID FLOW, ANDSAID SECONDARY CONTROL